Zinc oxide thin films were reactively deposited on glass substrates by r.f. magnetron-mode sputtering employing electron cyclotron resonance (ECR) plasma, and their crystallographic characteristics, electrical resistivity and optical properties were characterized. Single-phase zinc oxide thin films were deposited at very low gas pressures of 10(-2) Pa in an O-2 and Ar mixed gas atmosphere. They exhibited a c-axis orientation of below 3 degrees full width at half maximum (FWHM) for X-ray rocking curves, an extremely high resistivity of 10(7)-10(10) Omega cm, and a low optical attenuation of 3.3 dB cm(-1) at a wavelength of 0.63 mu m. These results indicate that irradiation of ECR plasma during deposition plays an important role in preparing high-quality zinc oxide film.